Theory of electrothermal behavior of bipolar transistors: part II-two-finger devices

A thorough theoretical and numerical analysis of the electrothermal behavior of two-finger bipolar transistors is presented. It is shown that thermal feedback and coupling effects introduce an additional singularity in the output I-V characteristics, namely a current bifurcation, which manifests itself as multiple solution branches emanating from a branching point. As a result, when the bifurcation condition is reached, the device is triggered in an asymmetrical operation mode in which one device carries most of the current. A unified formulation for the electrothermal behavior of a two-finger device is derived for different bias conditions at the input port: constant voltage, constant base current and constant emitter current. The analysis proofs that the critical condition defining the onset of current bifurcation is the same for all kinds of bias conditions. While operation under a constant base-emitter voltage is limited by the flyback condition, the current bifurcation condition defines the boundary of the normal operation region for the constant base current and constant emitter current cases. Finally, a rigorous method for identifying the conditions of thermal instability for an arbitrary number of emitter fingers is outlined. As an example, the method is used to derive the thermal instability conditions for the general case of temperature dependent thermal conductivity in a two-finger device.     

Analysis of the large-signal characteristics of powerheterojunction bipolar transistors exhibiting self-heating effects

The large-signal microwave characteristics of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) are modeled using the conventional Gummel-Poon-based bipolar junction transistor (BJT) model and extending it to include self-heating effects. The model is incorporated as a user-defined model in a commercial circuit simulator. The experimental microwave characteristics of HBTs are analyzed using the new model and harmonic balance techniques and the impact of self-heating effects on the device large-signal characteristics is investigated. Use of constant base voltage rather than constant current is more suitable for achieving maximum output power. Self-heating induced by RF drive is reduced under constant base current conditions. Increased thermal capacitance values result in gain enhancement at high power levels     

Bias dependence of high-frequency noise in heterojunction bipolar transistors

Hawkins' isothermal model developed to study noise in bipolar junction transistors (BJTs) is modified to investigate bias-dependent noise in heterojunction bipolar transistors (HBTs) by incorporating thermal effects. It is shown that the inclusion of thermal effects into the high-frequency noise model of HBTs is necessary as the temperature of the device may become very different from the ambient temperature, especially at high bias current. Calculation of the noise figure by including the thermal effect shows that the isothermal calculation may underestimate the noise figure at high bias current. It is observed that noise at low bias is ideality factor n dependent whereas high bias noise is insensitive to the variation of n. Moreover, the common base current gain plays a major role in the calculation of the minimum noise figure. The excellent fit obtained between the theoretical calculation and the measured data are attributed to the inclusion of the bias-dependent junction heating as well as C/sub De/ and C/sub bc/ into the present calculation.     

Diode ideality factor for surface recombination current inAlGaAs/GaAs heterojunction bipolar transistors

n-p-n AlGaAs/GaAs heterojunction bipolar transistors of various emitter areas have been fabricated to examine the diode ideality factor for surface recombination. These transistors are fabricated with and without exposed extrinsic base surfaces. Comparison of the measured results indicates that the base surface recombination current increases exponentially with the base-emitter voltage with an ideality factor which is closer to 1 than 2(1<n<1.33). This finding agrees with a published theoretical analysis of base surface recombination in HBTs. This study is compared with other experimental work     

Geometrical effects in high current gain 1100-V 4H-SiC BJTs

This paper reports the fabrication of epitaxial 4H-SiC bipolar junction transistors (BJTs) with a maximum current gain /spl beta/=64 and a breakdown voltage of 1100 V. The high /spl beta/ value is attributed to high material quality obtained after a continuous epitaxial growth of the base-emitter junction. The BJTs show a clear emitter-size effect indicating that surface recombination has a significant influence on /spl beta/. A minimum distance of 2-3 /spl mu/m between the emitter edge and base contact implant was found adequate to avoid a substantial /spl beta/ reduction.     

A curvature calibrated bandgap reference with base-emitter current compensating in a 0.13/tm CMOS process

In bandgap References,the effect caused by the input offset of the operational amplifier can be effectively reduced by the utilization of cascade bipolar junction transistors(BJTs).But in modern CMOS logic processes,due to the small value of β,the base-emitter path of BJTs has a significant streaming effect on the collector current,which leads to a large temperature drift for the reference voltage.To solve this problem,a base-emitter current compensating technique is proposed in a cascade BJT bandgap reference structure to calibrate the curvature of the output voltage to temperature.Experimental results based on the 0.13 μm logic CMOS process show that the reference voltage is 1.238V and the temperature coefficient is 6.2 ppm/℃ within the range of-40 to 125 ℃.     

An analysis of base bias current effect on SiGe HBTs

The anomalous dip in scattering parameter S/sub 11/ of SiGe heterojunction bipolar transistors (HBTs) is explained quantitatively for the first time. Our results show that for SiGe HBTs, the input impedance can be represented by a "shifted" series RC circuit at low frequencies and a "shifted" parallel RC circuit at high frequencies very accurately. The appearance of the anomalous dip of S/sub 11/ in a Smith chart is caused by this inherent ambivalent characteristic of the input impedance. In addition, it is found that under constant collector-emitter voltage (V/sub CE/), an increase of base current (which corresponds to a decrease of base-emitter resistance (r/sub /spl pi//) and an increase of transconductance (g/sub m/)) enhances the anomalous dip, which can be explained by our proposed theory.     

Time-dependent carrier flow in a transistor structure under nonisothermal conditions

A time-dependent temperature-dependent two-dimensional model has been developed to illustrate the internal behavior of bipolar transistors. Electrothermal interactions within the device are calculated in an attempt to better understand thermal instability modes. Numerically computed results are presented showing the electrical and thermal effects in the transistor operating in the switching mode during turn-on along a resistive load line. Plots of current density, electrostatic potential, and temperature versus time illustrate the combined effects of electrical rise time, thermal delay time, electrothermal interaction, current crowding, current spreading, conductivity modulation, and base pushout in bipolar transistors.     

Reliability of AlInAs/GaInAs heterojunction bipolar transistors

The reliability of high-performance AlInAs/GaInAs heterojunction bipolar transistors (HBTs) grown by molecular beam epitaxy (MBE) is discussed. Devices with a base Be doping level of 5×10¹⁹ cm^-3 and a base thickness of approximately 50 nm displayed no sign of Be diffusion under applied bias. Excellent stability in DC current gain, device turn-on voltage, and base-emitter junction characteristics was observed. Accelerated life-test experiments were performed under an applied constant collector current density of 7×10⁴ A/cm² at ambient temperatures of 193, 208, and 328°C. Junction temperature and device thermal resistance were determined experimentally. Degradation of the base-collector junction was used as failure criterion to project a mean time to failure in excess of 10⁷ h at 125°C junction temperature with an associated activation energy of 1.92 eV     

Turn-on voltage investigation of GaAs-based bipolar transistors with Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ base layers

The fundamental lower limit on the turn on voltage of GaAs-based bipolar transistors is first established, then reduced with the use of a novel low energy-gap base material, Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/. InGaP/GaInAsN DHBTs (x/spl sim/3y/spl sim/0.01) with high p-type doping levels (/spl sim/3/spl times/10/sup 19/ cm/sup -3/) and dc current gain (/spl beta//sub max//spl sim/68 at 234 /spl Omega///spl square/) are demonstrated. A reduction in the turn-on voltage over a wide range of practical base sheet resistance values (100 to 400 /spl Omega///spl square/) is established relative to both GaAs BJTs and conventional InGaP/GaAs HBTs with optimized base-emitter interfaces-over 25 mV in heavily doped, high dc current gain samples. The potential to engineer turn-on voltages comparable to Si- or InP-based bipolar devices on a GaAs platform is enabled by the use of lattice matched Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ alloys, which can simultaneously reduce the energy-gap and balance the lattice constant of the base layer when x/spl sim/3y.     

Current transient in polyimide-passivated InP/InGaAs heterojunction bipolar transistors: systematic experiments and physical model

We present the first comprehensive experimental investigation of base current transient in InP/InGaAs heterojunction bipolar transistors (HBTs) induced by polyimide passivation, and its influence on the device characteristics. We have confirmed that the trapping and detrapping of static charge in the polyimide at the polyimide/semiconductor interface is the root cause for the current transient effect. Experimental and theoretical evidence indicates that the base current transient is caused by the change of the base-emitter junction surface potential as a result of the decrease in the number of trapped electrons in the polyimide film. A physical model accounting for the time-dependence of surface recombination current induced by electron detrapping in the polyimide is derived and predicts the current transient behavior. The physical understanding is helpful for further optimization of polyimide passivation process for HBTs.     

Temperature dependent common emitter current gain andcollector-emitter offset voltage study in AlGaN/GaN heterojunctionbipolar transistors

We have demonstrated state-of-the-art performance of AlGaN/GaN heterojunction bipolar transistors (HBTs) with a common emitter (CE) current gain of 31 at 175 K and 11.3 at 295 K. The increase in collector current and CE current gain at lower temperature can be attributed to the reduced base-emitter interface recombination current. We also observed an increase of collector-emitter offset voltage with decrease of temperature. The increase of V_CEOFF at lower temperature is related to an increase of V_BE as the base bulk current is increased, or to the reduction of the ideality factor n_BE     

Failure mode analysis of AlGaAs/GaAs HBTs using electrostatic discharge method

The failure modes of AlGaAs/GaAs heterojunction bipolar transistors (HBTs) showing increases of base leakage current found during the bias and temperature reliability test have been examined and analysed by using an electrostatic discharge (ESD) method. In the examination for HBT having an emitter of 2 × 20 μm, increases of a base leakage current have been observed when an ESD of around 100 V is applied through a base-emitter junction. Furthermore, increases of a collector leakage current have been observed with the additional ESD of around 150 V. Since no fatal effects for the transistor amplification are found in the high current region, the degradations are concluded to be responsible to the local damages of the base layer at the edge of base-emitter junction. Applying the Wunsch-Bell model to the damaged region, the temperature is estimated to be in a range from 400 to 550°C. It is thought that thermal destruction occurs in a base layer.     

An improved SPICE model for high-frequency noise of BJTs and HBTs

An improved compact model for the high-frequency noise of bipolar junction transistors (BJTs) and heterojunction bipolar transistors (HBTs) is presented and implemented in SPICE3. It properly takes into account the frequency function of thermal noise in the input circuit, which is related to the real part of the input admittance. This quantity is the result of an interplay between the base resistance, the internal emitter-base capacitance, the small-signal input conductance of the intrinsic transistor, and the emitter series resistance. This requires the replacement of the noisy base resistance in SPICE by a frequency-dependent expression consisting of the appropriate SPICE parameters. A similar substitution is needed in the output circuit. For a Si/SiGe HBT these improvements lead to excellent agreement between the noise figure and device simulation results     

Current-temperature feedback effects in III-V heterojunctionbipolar transistors

The consequence of the reciprocal relation between the temperature and current distributions in heterojunction bipolar transistors (HBTs) has been determined. The dc current voltage (I-V) characteristics, RF small-signal parameters, and temperature distributions of discrete devices with emitter fingers of varying lengths were analyzed empirically and their thermal profiles calculated numerically. The lateral temperature gradient induced in the finger due to power dissipation under normal operating conditions is shown to directly affect the current distribution in the transistor. The negative temperature dependence of the HBT base-emitter junction turn-on voltage results in positive feedback between current and temperature. This current temperature relationship leads to higher localized current densities in the hottest portion of the device, the center of the emitter. The temperature of the hot section rises with increasing power dissipation, continually drawing more current. Ultimately, the current through HBTs is localized to a comparable area at the finger center, independent of the emitter length     

微波功率SiGe HBT研究进展

Si的热导率比大部分化合物半导体(如GaAs)的热导率高,SiGe HBT在一个较宽的温度范围内稳定,SiGe HBT的发射结电压VBE的温度系数dVBE/dT比Si的小,双异质结SiGe HBT本身具有热-电耦合自调能力,所加镇流电阻可以较小,所有这些使SiGe HBT比GaAs HBT和SiBJT在功率处理能力上占一定优势。文章对微波功率SiGe HBT一些重要方面的国内外研究进展进行评述,希望对从事微波功率SiGe HBT的研究者有所帮助。     

Base Charge Dynamics of Abrupt Base–Emitter Junction HBTs and Its Representation in Transistor Models

Base charge dynamics in abrupt heterojunction bipolar transistors (HBTs) are determined by the thermionic/field emission boundary condition. As a result, their small and large signal models differ from those of bipolar junction transistors (BJTs), which obey the Shockley boundary condition. There is comparison of the base charge dynamics in the abrupt HBT to the BJT case and derive the small and large signal models of the abrupt HBT     

Thermal behaviour of 6H-SiC bipolar transistors: spice simulation and applications

As material quality improves and growth technology develops, SiC BJTs are regaining interest. They have the advantage of carrier modulation, high current capabilities and lower initial voltage drop. In this work, the thermal behaviour of 6H-SiC bipolar transistors is simulated. The examined figures of merit such as input resistance h₁₁, current gain β and transconductance g_m show superior performance of 6H-SiC BJTs, at high temperatures, when compared to similar silicon counterparts. In the range of temperatures −20 to 160 °C, drawbacks found in Si BJTs are attenuated or eliminated with the use of SiC BJTs. These advantages are transferred to 6H-SiC based circuits. The built current mirror shows quasi-ideal behaviour while the designed input stage of the amplifier has a voltage gain thermally stabilised up to 140 °C.     

Significant operating voltage reduction on high-speed GaAs-based heterojunction bipolar transistors using a low band gap InGaAsN base layer

We report the fabrication of double heterojunction bipolar transistors (DHBTs) with the use of a new quaternary InGaAsN material system that takes advantage of a low-energy band gap E/sub G/ in the base to reduce operating voltages in GaAs-based electronic devices. InGaP/In/sub 0.03/Ga/sub 0.97/As/sub 0.99/N/sub 0.01//GaAs DHBTs with improved band gap engineering at both heterojunctions exhibit a DC peak current gain over 16 with small active emitter area. The use of the lattice-matched In/sub 0.03/Ga/sub 0.97/As/sub 0.99/N/sub 0.01/ (E/sub G/=1.20 eV) base layer allows a significant reduction of the turn-on voltage by 250 mV over standard InGaP/GaAs HBTs, while attaining good high-frequency characteristics with cutoff frequency and maximum oscillation frequency as high as 40 GHz and 72 GHz, respectively. Despite inherent transport limitations at the present time, which penalize peak frequencies, this novel technology provides comparable RF performance to conventional devices with a GaAs control base layer but at much lower operating base-emitter bias conditions. This technical progress should benefit to the next generation of RF circuits using GaAs-based HBTs with lower power consumption and better handling of supply voltages in battery-operated wireless handsets.